The invention relates generally to auto-correlation filters. More particularly, the invention relates to auto-correlation filters that can operate in a continuous real-time manner.
Auto-correlation filters are known and operate on the basic principle that synchronous signals can be discriminated from asynchronous signals or noise by summing repeated cycles of the signal. When such summation is performed over a relatively large number of cycles, the asynchronous signals average out to near zero, while the synchronous signal of interest remains at its average level.
As an example, ultrasonic sensor systems that use echo ranging produce output signals that include a true target echo and random noise signals and false echoes. In many cases, such as a liquid level sensor for example, a large number of transmit/receive cycles can be performed and the true echoes are substantially synchronous with each other, but may be small in magnitude compared to associated random noise from the system. Use of an auto-correlation filter can substantially improve the output signal-to-noise ratio.
Known auto-correlation filters typically operate on the basis of digital signal processing under the control of a microprocessor. The analog signals are digitized and then stored in digital memory devices. The multiple signals are then processed in digital form because of the need to retain the data for purposes of averaging. Such digital signal processing has numerous drawbacks. One drawback is that such signal processing demands high power digital processing usually realized with complicated software and a microprocessor controller. The use of digital memory storage also limits the ability to operate the filter in a real-time mode because the system must sample and store a number of data cycles and then interrupt data collection in order to perform the digital signal processing. After the auto-correlation process is completed, data collection is started over.
The objective exists, therefore, for auto-correlation filtering methods and apparatus that can operate in continuous real time without using microprocessor control circuits or other digital signal processing.